Plasma etching method

ABSTRACT

An etching method for forming a recess ( 220 ) having an opening dimension (R) of millimeter order in an object ( 212 ) to be etched such as a semiconductor wafer. A mask ( 214 ) having an opening corresponding to the recess ( 220 ) is formed on the object ( 212 ). The object ( 212 ) with the mask ( 214 ) is placed in a processing vessel for plasma etching and etched in it using a plasma. The material of the portion around the opening of the mask ( 214 ) is the same as the material, for example, silicon of the object ( 212 ). Hence, the recess ( 220 ) can be so formed as not to form a sub-trench shape (a shape formed by etching the periphery of which is deeper than the center) substantially in the bottom ( 222 ).

FIELD OF THE INVENTION

The present invention relates to a method for etching an object to beetched, e.g., a semiconductor wafer, through a mask by using a plasma.

BACKGROUND OF THE INVENTION

Conventionally employed as an etching method is a plasma etching foretching an object to be etched by using a plasma. Employed in such aplasma etching method is, for example, an etching apparatus including alower electrode and an upper electrode disposed opposite to face eachother within a hermetically sealed processing vessel. In the etchingapparatus, an object to be etched having a mask formed on a surfacethereof is mounted on the lower electrode and a processing gas issupplied into the processing vessel. Thereafter, a high frequency poweris applied to the lower electrode to excite the plasma, therebyexecuting the plasma etching.

The plasma etching method is widely employed to form a predeterminedpattern on a surface of an object to be etched such as a semiconductorwafer and a liquid crystal substrate. The pattern thus formed has a sizeof the order of micron, in general.

Recent diversification in technical field will make it almost surely anecessity to form a recess with an opening dimension of the order ofmillimeter in the course of, for instance, microfabricating a jig forthe formation of an integrated circuit.

FIG. 9 provides a schematic cross sectional view of an object to beprocessed 10 having a recess with an opening dimension of the order ofmillimeter which is formed by the conventional etching method. Theobject to be processed 10 shown in FIG. 9 is obtained by way of etchinga semiconductor wafer 12 to be etched in accordance with theconventional etching method by using a desired pattern previously formedon a photoresist 14 provided on the semiconductor wafer 12 as a mask. Arecess 20 thus formed in the wafer 12 is of a substantially cylindricalshape and its opening dimension (diameter) R ranges, for example, from10 to 30 mm.

When the recess 20 with the opening dimension of the order of millimeteris formed by the conventional etching method as described above, aperipheral portion 24 of a bottom surface of the recess 20 may happen tobe etched deeper than a central portion 22 thereof (hereinafter referredto as a “subtrench shape”). However, in most applications, it ispreferable that the bottom surface of the recess is flat. In fact, thesubtrench shape of the recess impedes a precision fabrication of variousdevices.

SUMMARY OF THE INVENTION

It is, therefore, an object of the present invention to provide anetching method capable of forming a recess with an opening dimension ofthe order of millimeter without generating a subtrench shape on a bottomsurface thereof.

In accordance with the present invention, there is provided an etchingmethod for forming a recess on a surface of an object to be etched,including the step of forming the recess by plasma etching through anopening of a mask formed on the surface of the object to be etched,wherein at least a portion of the object to be etched where the recessis to be formed and at least a portion of the mask around the openingare all made of silicon.

In accordance with the present invention there is further provided anetching method for forming a recess on a surface of an object to beetched, including the step of forming the recess by plasma etchingthrough an opening of a mask formed on the surface of the object to beetched, wherein the opening includes a main opening portioncorresponding to the recess to be formed and a slit-shaped subsidiaryopening portion surrounding the main opening portion.

In accordance with these etching methods, a recess with an openingdimension of the order of millimeter can be obtained without a subtrenchshape formed at a bottom surface thereof.

In accordance with the present invention, there is further provided anetching method for forming a recess on a surface of an object to beetched, including the step of forming the recess by plasma etchingthrough an opening of a mask formed on the surface of the object to beetched, the opening having an opening dimension not smaller than 5 mm,wherein at least a portion of the object to be etched where the recessis to be formed and at least a portion of the mask around the openingare all made of a same material.

In accordance with the present invention, there is further provided anetching method for forming a recess on a surface of an object to beetched, including the step of forming the recess by plasma etchingthrough an opening of a mask formed on the surface of the object to beetched, the opening having an opening dimension not smaller than 5 mm,wherein the plasma etching is performed in a processing vessel at apressure not greater than 100 mTorr.

In accordance with these etching methods, a recess with an openingdimension greater than 5 mm can be obtained without a subtrench shapeformed on a bottom surface thereof.

The opening dimension refers to a length of a shorter side in case theopening (opening edge portion of the recess) is of a substantiallyrectangular shape; a diameter in case of a circular shape; a shorterdiameter in case of an elliptic shape; and a width in case of a grooveshape (this is same in a description as will be described later).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross sectional view of an etching apparatus foruse in performing an etching method in accordance with the presentinvention;

FIG. 2 provides a schematic cross sectional view showing an ideal shapeof a recess to be formed on an object to be etched;

FIGS. 3A and 3B set forth a plane view and a partial cross sectionalview of an object to be processed, respectively, for explaining aconventional etching method;

FIG. 4 shows a result of investigating a shape of a recess formed by theconventional etching method by means of a surface profiler;

FIGS. 5A and 5B illustrate a plane view and a partial cross sectionalview of an object to be processed, respectively, for explaining anetching method in accordance with a first preferred embodiment of thepresent invention;

FIGS. 6A and 6B provide results of investigating a shape of a recessformed by the method in accordance with the first embodiment explainedby using FIGS. 5A and 5B in an X-direction and a Y-directionrespectively by means of the surface profiler;

FIGS. 7A and 7B offer a plane view and a cross sectional view of anobject to be processed, respectively, for explaining an etching methodin accordance with a second preferred embodiment of the presentinvention;

FIG. 8 is a schematic cross sectional view of an object to be processedetched by the method in accordance with the second embodiment describedin FIG. 7; and

FIG. 9 depicts a schematic cross sectional view of an object to beprocessed by the conventional method.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiment of the present invention will now be described indetail with reference to the accompanying drawings, wherein like partswill be assigned like reference numerals and explanations thereof willnot be repeated.

First Preferred Embodiment

Referring to FIG. 1, there is illustrated a schematic cross sectionalview of an etching apparatus 100 for use in performing an etching methodin accordance with the present invention. The etching apparatus 100includes a processing vessel 102 for accommodating therein asemiconductor wafer W to be processed having a mask formed on a surfacethereof. The processing vessel 102 is grounded and made of aluminumhaving an aluminum oxide film layer formed on the surface thereof by,e.g., anodic oxidation.

Disposed within the processing vessel 102 is a lower electrode 104 alsoserving as a susceptor for mounting thereon the wafer W to be processed.The lower electrode 104 is covered with an insulating member 105 madeof, e.g., ceramic and a conductive member 107 formed of, e.g., aluminum,except a mounting surface thereof.

The lower electrode 104 is moved up and down by an elevating shaft 106.Further, provided between the conductive member 107 and the processingvessel 102 is a bellows 109 which is made of, e.g., a stainless steel.Further, since the aluminum oxide film layer is removed from surfaceportions of the conductive member 107 and the processing vessel 102where they are making an electrical contact with the bellows 9, theconductive member 107 is grounded via the bellows 109 and the processingvessel 102. Further, a bellows cover 111 is disposed to surround alateral surface of the conductive member 107 and the bellows 109.

Provided on the mounting surface of the lower electrode 104 is anelectrostatic chuck 110 which is connected to a high voltage DC powersupply 108. And, a focus ring 112 is disposed to surround theelectrostatic chuck 110. Connected to the lower electrode via a matchingunit 116 is a high frequency power source 118 for supplying a highfrequency power.

Provided at a lateral side of the lower electrode 104 is a baffle plate120 interposed between the focus ring 112 and the conductive member 107,wherein the baffle plate 120 is fixed on a top portion of the conductivemember 107 via one or more conductive screws (not shown). The baffleplate 120 is made of a conductive material, e.g., aluminum whose surfaceis treated by anodic oxidation. However, the aluminum oxide film layerof the baffle plate 120 is partially removed at joint portions where thebaffle plate 120 and the conductive member 107 make an electricalcontact with each other.

Accordingly, the baffle plate 120 is grounded via the conductive member107, the bellows 109 and the processing vessel 102 and, thus, the baffleplate 120 and an inner wall of the processing vessel 102 can be set tohave a substantially same electric potential (ground potential). As aresult, the baffle plate 120 and an upper portion of the inner wall ofthe processing vessel 102 above the baffle plate 120 can serve asopposing electrodes of the lower electrode 104. By such configuration, aplasma can be confined within a space above the baffle plate 120, i.e.,in a processing space 122 to be described later.

The baffle plate 120 is provided with a plurality of slits 120 a andserves to divide the interior of the processing vessel 102 into theprocessing space 122 in which the wafer W is disposed and a gas exhaustpath 124 which communicates with a gas exhaust line 128 to be describedlater.

An upper electrode 126 is installed in the upper interior surface of theprocessing vessel 102 to face the mounting surface of the lowerelectrode 104. The upper electrode 126 is provided with a plurality ofgas injection openings 126 a through which a processing gas is suppliedinto the processing space 122 from a gas supply source (not shown).

Connected to a lower portion of the processing vessel 102 is the gasexhaust line 128, which in turn is coupled to a vacuum exhaust unit (notshown). The atmosphere within the processing space 122 is evacuated to apredetermined vacuum level via the slits 120 a of the baffle plate 120,the gas exhaust path 124 and the gas exhaust line 128. Further, a magnet130 is disposed to an exterior of the processing vessel 102 to surrounda plasma area formed in the processing space 122.

In the following, there will be explained exemplary etching conditions.

A wafer made of silicon (Si) with a diameter of 200 mm is employed as anobject to be etched. An SF₆ gas or a gaseous mixture of SF₆ and O₂ canbe used as a processing gas. Specifically, a gaseous mixture of the SF₆gas having a flow rate ranging from 200 to 1000 sccm (e.g., 800 sccm)and the O₂ gas having a flow rate not greater than 300 sccm (e.g., 150sccm) can be utilized. An inner pressure of the processing vessel 102 isset to range from 200 mTorr to 400 mTorr. A temperature of the mountingsurface of the lower electrode 104 is determined to be between −15° C.to 10° C. inclusive. Further, a high frequency power ranging from 500 Wto 2000 W with 40 MHz is applied to the lower electrode 104. Thestrength of a magnetic field formed by the magnet 130 is set to be 170gausses.

Referring to FIG. 2, there is provided a schematic cross sectional viewshowing an ideal shape of a recess to be formed in an object to beetched. An object 200 to be processed shown in FIG. 2 includes a wafer212 to be etched which is made of silicon; and a mask 214 formed on thewafer 212. The mask 214 has an opening corresponding to a recess 220 tobe formed. By etching the object 200 to be processed through the use ofthe plasma etching apparatus 100 described in FIG. 1, the recess 220having a substantially vertical wall and a flat bottom surface 222 isformed on a surface of the wafer 212, as shown in FIG. 2.

FIGS. 3A and 3B provide a plane view and a partial cross sectional viewof an object 10 to be processed, respectively, for describing aconventional etching method. The object 10 to be processed shown inFIGS. 3A and 3B includes a wafer 12 to be etched which is made ofsilicon; and a mask 14 formed on the wafer 12 as well. The mask 14 hasopenings 16 corresponding to recesses to be formed. In the conventionaletching method, the material for the mask 14 is a tape made of apolyimide based polymer material. Further, the opening of the mask 14 isof a square shape with a side length (opening dimension) R1 of 30 mm,and the thickness D of the mask 14 is 25 μm.

The object 10 to be processed was plasma-etched under theabove-specified processing conditions by using the etching apparatus 100shown in FIG. 1. FIG. 4 shows a result of investigating a shape of therecess thus formed by using the surface profiler along with an etchingrate (E/R) at each portion. In FIG. 4, a horizontal axis corresponds toan X direction of FIG. 3A and measurements were conducted from an outerend of the recess, which was closer to a periphery of the object 10 tobe processed, toward an inner end of the recess opposing to the outerend. As can be seen from FIG. 4, the etching rates (μm/min) at the outerend and the inner end were found to be 5.6 and 6.8, respectively, whichwere far greater than the etching rate of 3.6 at a central portion ofthe recess. Such a difference in the etching rates results in asubtrench shape.

It is conjectured that such a result is due to the fact that since therecess has an opening whose dimension is beyond the order of millimeter,a plasma existing right above the recess (opening) becomes stronglyinfluenced by the silicon of the object to be etched which is differentfrom material making up the mask, thereby converting it into a statedifferent from that of a plasma formed above the mask. That is, it isconsidered that a region around the outer end of the recess willexperience an etching rate higher than that of the central portion dueto the influence from the plasma present above the mask.

FIGS. 5A and 5B illustrate a plane view and a partial cross sectionalview of an object 200 to be processed, respectively, for explaining theetching method in accordance with the present invention.

As can be inferred from the foregoing description, it is preferable thatat least a portion around the opening of the mask is made of the samematerial as that used for forming the wafer in case of forming therecess in the wafer to be etched. By using the same material, it isconsidered that the plasma formed above the object to be processed canbe made uniform.

For a comparison with the conventional etching method, the object 200 tobe processed was prepared as shown in FIGS. 5A and 5B. Specifically, theobject 200 to be processed included a wafer 212 made of silicon, whereinformed on the wafer 212 as a mask were walls of silicon chips 214 to beoutwardly adjacent to two facing X-directional end portions of anopening 216 corresponding to a recess to be formed; and a tape type mask218 made of a conventional polyimide based polymer material formed inregions on the wafer 212 other than where the silicon chips 214 and theopening 216 were located. Accordingly, only the portions adjacent to theX-directional facing end portions of the opening 216 could be formed ofthe same material as that of the object 212 to be etched.

Herein, the opening 216 of the mask is of a square shape with a sidelength R1 of 30 mm, and the thickness D of the mask 218 is 25 μm.Further, the height H of each silicon chip 214 is 725 μm and the widthW1 thereof is 10 mm.

Etching of the object 200 to be processed was conducted under theabove-described processing conditions by employing the etching apparatus100 shown in FIG. 1. FIGS. 6A and 6B show a result of investigation on ashape of the recess thus obtained by using the surface profiler alongwith an etching rates at each portion. A horizontal axis of FIG. 6Acorresponds to an X direction of FIGS. 5A and 5B while a horizontal axisof FIG. 6B corresponds to a Y direction thereof. In both of FIGS. 6A and6B, measurements were conducted from an outer end of the recess, whichwas closer to a periphery of the object 200 to be processed, toward aninner end thereof, which was located opposite to the outer end. As canbe seen from FIG. 6B corresponding to the conventional etching method (Ydirection), the etching rates (μm/min) at the outer end and the innerend were found to be 4.7 and 5.7, respectively, which were clearlygreater than the etching rate of 2.9 at a central portion of the recess.Apparently, such a difference in the etching rates must have resulted inthe subtrench shape.

Meanwhile, in FIG. 6A corresponding to the etching method of the presentinvention (X direction), there was observed no subtrench shape and,further, the etching rate (μm/min) at a central portion of the recesswas found to be 3.3, not making any noticeable difference from that inFIG. 6B. The achievement is considered possible because the state of theplasma above the recess (opening portion) can be uniformly maintained,even in case the recess has a large opening dimension, by forming atleast the portion around the opening of the mask with the same materialas used for forming the object to be etched.

As described above, by forming at least the portion around the openingof the mask with the same material as used for the formation of theobject to be etched, generation of a subtrench shape can be preventedfrom being developed even in case of forming, e.g., a recess of arectangular column with an opening dimension of 30 mm, so that therecess with a substantially flat bottom surface can be obtained. Thus,it becomes possible to form the recess with the opening size of theorder of millimeters without a subtrench shape formed at the bottomsurface thereof.

Furthermore, if a portion of the object to be etched where the recess isto be formed is made of a material different from that forming the otherportions of the object, it is preferable to form at least the portionaround the opening of the mask by using the same material as used forforming the portion where the recess is to be formed.

Second Preferred Embodiment

An etching method in accordance with a second preferred embodiment ofthe present invention can be performed by using the etching apparatus100 (shown in FIG. 1) as described in the first embodiment. Thus,explanation thereof will be omitted herein.

Referring to FIGS. 7A and 7B, there are provided a plane view and across sectional view of an object 300 to be processed, respectively, fordescribing the etching method in accordance with the second embodiment.Further, FIG. 8 shows a schematic cross sectional view of the object 300etched in accordance with the second embodiment.

The object 300 to be processed includes a wafer 312 to be etched whichis made of silicon and a mask 314 which is formed on the wafer 312. Thematerial for the mask 314 is a tape made of a polyimide based polymermaterial. The mask 314 has a main opening 316 corresponding to a recessto be formed and a slit-shaped subsidiary opening 324 closelysurrounding the main opening 316. Thus, a dummy recess corresponding tothe subsidiary opening 324 is formed on a surface of the wafer 312through the etching of the object 300 to be processed, the dummy recessbeing disposed around the recess corresponding to the main opening 316.

Herein, the main opening 316 of the mask 314 is of a square shape with aside length R1 of 30 mm, and the thickness D of the mask 14 is 25 μm.Further, the slit width W2 of the subsidiary opening 324 is 5 mm and awidth W3 of a mask portion 318 between the openings 316 and 324 is 100μm.

Plasma etching of the object 300 to be processed is conducted under theabove-described processing conditions by employing the etching apparatus100 shown in FIG. 1. As a result, the dummy recess corresponding to thesubsidiary opening 324 will have a slant bottom surface 332 tilted downtoward an outer peripheral side thereof, as shown in FIG. 8, which lookssimilar to the subtrench shape. The formation of the slant bottomsurface 332 of such a shape is conjectured as a result of an increase ofan etching rate at the outer peripheral side of the recess 330 due tothe influence of the material of the mask 314 on the plasma.

In contrast, no subtrench shape is formed on a bottom surface 322 of arecess 320 corresponding to the main opening 316, and its bottom surface322 is maintained flat. It seems that if the width of the mask portion318 is very narrow, the influence of the mask material on the plasmaabove the recess 320 can be minimized. That is, it is likely that plasmaaffected by the silicon of the wafer 312 exposed through the subsidiaryopening 324 is formed above the opening edge portion of the recess 320,so that the plasma present above the recess 320 can be made uniform.

By using the mask 314 having the main opening 316 and the subsidiaryopening 324 described, formation of a subtrench shape can be preventedfrom being developed even in case of forming, e.g., the recess 320 ofthe rectangular column shape with the opening dimension of 30 mm,thereby obtaining a recess having the flat bottom surface. Accordingly,it becomes possible to fabricate a recess with an opening dimension ofthe order of millimeter without forming a subtrench shape on the bottomthereof.

Third Preferred Embodiment

Since an etching method in accordance with a third preferred embodimentof the present invention can be performed by using the etching apparatus100 provided with the processing vessel 102 (shown in FIG. 1) asdescribed in the first embodiment, explanation thereof will be omittedherein.

In the third embodiment, the inner pressure of the processing vessel 102is set to be lower than that in conventional cases during etching. Thatis, though the inner pressure of the processing vessel 102 is set torange from 200 to 400 mTorr in the first and the second embodiment, theinner pressure in the third embodiment is determined to be not greaterthan 100 mTorr, e.g., 36 mTorr. The other processing conditions areidentical to those in the first and the second embodiment.

Further, a mask made of, e.g., a general resin as in the case of theconventional object 100 to be processed shown in FIG. 3 can be used foran object to be processed for use in the third embodiment. In accordancewith this embodiment, a recess having a substantially vertical sidewalland a flat bottom surface, which is similar to the recess 220 shown inFIG. 2, can be formed even though the conventional object to beprocessed is etched.

If plasma etching is carried out under the pressure lower than in theconventional cases as in the third embodiment, a plasma density withinthe processing space 122 (FIG. 1) is reduced. Though an overall etchingrate is lowered in such a case when compared with a case featuring ahigh plasma density, it is unlikely that there occurs a difference inetching rates due to the influence of materials forming the surface ofthe object to be processed. It is conjectured that the decrease in theplasma density in turn reduces influences of the materials of thesurface of the object to be processed on the status of the plasmaexisting thereabove.

Thus, it becomes possible to form a recess with an opening dimension ofthe order of millimeter without a subtrench shape formed on a bottomsurface thereof. Furthermore, since the third embodiment can be carriedout by using a mask having a conventional shape or made of aconventional material, the mask formation can be achieved by employingthe same technique as adopted in the conventional cases.

Further, the present invention is not limited to the preferredembodiment described. While the invention has been shown and describedwith reference to the preferred embodiments, it will be understood bythose skilled in the art that various changes and modifications may bemade without departing from the spirit and scope of the invention asdefined in the following claims.

For example, though the present invention has been described for thecase of forming the recess of the quadrangular pyramid shape with theopening dimension of 30 mm in the preferred embodiments, the shape andthe dimension of the recess to be formed is not limited thereto. Thatis, the present invention can be applied to etching of a groove or arecess with a cross section of various shapes, e.g., a circular shape,an elliptic shape, a rectangular shape, a groove shape, etc., as long asthe recess has an opening dimension of the order of millimeter (forexample, not smaller than 5 mm). Moreover, the material for the objectto be processed is not limited to silicon. That is, the presentinvention can also be applied to etching of an object to be processed atleast partially substituted with a material other than silicon.

1. An etching method for forming a recess on a surface of an object tobe etched, comprising the step of forming the recess by plasma etchingthrough an opening of a mask formed on the surface of the object to beetched, wherein the opening of the mask includes a main opening portioncorresponding to the recess to be formed and a slit-shaped subsidiaryopening portion surrounding the main opening portion, the slit width ofthe subsidiary opening portion being narrower than the width of the mainopening portion.
 2. The method of claim 1, wherein the main opening ofthe mask has an opening dimension not smaller than 5 mm.